In this study, we examined whether and how the cellular activity of interleukin-2 (IL-2) is affected by glutathione (GSH), an important tripeptide existing in most cells. Cell culture and thymidine incorporation assay showed that addition of GSH enhanced the effect of IL-2 on the proliferation and thymidine incorporation of IL-2 dependent cytotoxic T-cells such as CTLL-2 and CT-4R. Treatment of the cells with GSH resulted in a two-fold increase in the amount of IL-2 bound to the cells and a rapid internalization of the bound IL-2. In addition, the degradation of IL-2 in the cells was enhanced by GSH treatment. These effects of GSH were accompanied by an increase in the intracellular GSH level. L-Buthionine-(S,R)-sulfoximine, an inhibitor of de novo GSH synthesis, blunted the increase of intracellular GSH level and modulated the effect of GSH on IL-2 activity. These results suggest that GSH regulates the binding, internalization, degradation, and T-cell proliferative activity of IL-2. To examine how GSH regulates the binding, internalization and degradation of IL-2, we used Northern blot and slot blot to analyze mRNA of IL-2Rp55 and IL-2rP70 which are the two major components of the high affinity IL- 2 receptors. We found that the IL-2Rp55 mRNA level reached maximum 4 h after GSH treatment and then declined, whereas the IL-2Rp7O mRNA level increased 1 h after GSH treatment and then remained relatively constant. GSH treatmen shortened the half-life (from 5 h to less than 3 h) and increased the turnover of surface high affinity IL-2 receptors. Receptor reappearance study, on the other hand, showed that GSH treatment did not increase the recycling of existing IL-2 receptors. These results suggest that GSH may potentiate the binding, internalization and degradation of IL-2 by enhancin the expression and turnover but not the recycling of IL-2 receptors.